Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
irrīsĭo (inr-), ōnis, f. [irrideo], a deriding, mocking, mockery: liber a tali irrisione Socrates, Cic. Ac. 2, 39, 123: stultitiae, Auct. Her. 1, 6, 10: irrisio omnium, Cic. de Or. 1, 12, 50: cum irrisione audientium, id. de Off. 1, 38, 137.